The present invention relates to a traction slip and brake slip-controlled brake system for automotive vehicles with a driven and a non-driven axle.
A brake system of this type is disclosed, for example, in published German patent document DE-OS No. 34 07 538, which describes a dual circuit brake system of diagonal division for an automotive vehicle having a driven front axle. The wheels of a diagonal, for example, the front-left wheel and the rear-right wheel, are jointly controlled by an outlet and an inlet valve. The outlet valve establishes communication with a reservoir whereas the inlet valve controls the communication with a pressure source.
In brake slip control, a connection is established to a pressure source in which a pressure proportional to the pedal force is generated. This pressure may be generated by a pump the pressure of which is determined by the master cylinder pressure, or by a brake pressure control valve reducing an accumulator pressure to a pressure in proportion to the pedal force. In traction slip control, the brake circuits are connected to a pressure source generating a pressure independently of the pedal force. Significantly, in traction slip control, the connection of the first pressure source, which generates a pressure in proportion to the pedal force to the brake circuits, is interrupted. Because of this, the second pressure source generating a pressure independent of the pedal force would be ineffective as there would be a connection to the non-pressurized reservoir through the non-actuated first pressure source.
Moreover, a separating valve is provided in the branch conduit leading to the brakes of the non-driven axle, such as the rear axle, which separating valve blocks the connection in the event of traction slip control. This is required because the non-driven wheels in the start-up process must not be pressure-loaded. In the prior art, this valve generally takes the form of an electromagnetically driven 2-way/2-position valve. Valves of this type exhibit a number of disadvantages in that they are expansive and permanently loadable only to a limited extent. This is, they are energizable for only a short period of time. The valve must be blocked during the entire period of a traction slip control. This means that during that time electrical current flows through the energizing coils with the result that the valve is excessively heated with the result that the valve no longer works properly as heat expansion causes jamming of parts of the valve. Accordingly, a hydraulic drive of the valves rather that an electromagnetic one has been resorted to.